1. Field of the Invention
The present invention is directed to an apparatus for storing an increased length of continuous tape medium in reeled form in standardized housing formats, such as dual reel cassettes and cartridges in rectangular format and in quadratic format.
2. Description of the Prior Art
The need for storing all types of information in a digital form is increasing very rapidly. Such information can be all types of computer based/generated data, data generated by document scanners, loggers, various form of video or audio, or a combination of any of these.
Conventionally, computers use magnetic hard disks to store such information which is rapidly required (short retrieval time), while tape is used extensively for backup of such hard disks and also for long term archival storage.
Tape is by far the cheapest high capacity media available today, and the suppliers of tape drives are constantly bringing out new drives with better performance (higher capacity, shorter access time, higher data rates etc.)
Originally, tape drives were based upon the open-reel principle: The tape to be recorded was supplied on one reel which was mounted on the tape drive by the operator. The tape was then guided from the supply reel across the recording head to an empty take-up reel. As the tape drive was running, more and more tape was moved from the supply reel to the take up reel. At the end, the tape was rewound from the take up reel back on to the supply reel. Then the operator could remove the supply reel and replace it with another one.
This system is still in use in some data systems today. The requirement for a trained operator, the size of the supply reel (normally 10 inches or more in diameter), and no protection of the tape when the supply reel was removed from the tape drive, make such open reel systems less attractive today. Instead, manufacturers and users have turned to various forms of cassette and cartridge based tape systems.
There are many tape cassette/cartridges in use today; however, they can typically be categorized into three basic groups: single reel cassette, dual reels cassette, and belt driven cassette or cartridge.
The single reel cassette is really a refined version of the original open-reel system. Well known examples of the single reel cassette, are the IBM 3480 (later enhanced into 3490 and newer models) and the DLT cassette (previously known as the Compact reel). These cassettes are characterized by a square (normally quadratic or close to quadratic) housing containing just one single tape cassette (with tape). During operation, the cassette is inserted in the drive and one end of the tape is automatically pulled out of the cassette and onto a take up reel inside the drive. The operation therefore resembles the old open reel system, except that the tape cassette is physically smaller, and designed so that loading and tape extraction can be done without operator involvement.
These cassettes have an opening on one side of the housing to allow the tape end to be pulled out during insertion into the tape drive. At the bottom of the cassette housing is an opening which allows a drive motor built into the tape drive to engage to the tape reel in the cassette.
These single reel cassettes have increased in popularity during the last few years, because they offer relatively large tape length in a fairly small cassette housing. Also, the cost of building tape drives which can handle such cassettes effectively and reliably has decreased considerably.
Nevertheless, single reel tape cassettes have some basic drawbacks. The most important one is that the tape always needs to be pulled out of the cassette onto the take up reel inside the drive. This requires a fairly sophisticated mechanism, and takes time. Many single reel cassette systems need between 20 and 60 seconds or more just to load the tape properly. This will significantly reduce the effective speed of the system (longer data access time).
Furthermore, if power is lost during operation, it is normally impossible to eject the cassette from the tape drive. For some application and systems, this may be a severe drawback.
FIG. 1 shows the basic principle design of a conventional single reel cassette. The fact that the cassette housing contains only one single reel of tape, makes it possible to achieve a large tape area in a fairly small cassette. In FIG. 1, 1 references the cassette housing. 2 references the tape reel containing the tape wound on hub 5, while 3 references the leader block (the beginning of the tape with a plastic hook that connects to the tape drive). The cassette housing 1 has a door 4 which opens to allow the leader block 3 to be pulled out.
The dual reel cassette is well known in many different versions: 4 mm DAT or DDS cassette, 8 mm videocassette, the Philips audio (Compact) cassette and the VHS video cassette just to name a few.
The principle of the dual reel cassette is that both tape reels are located within the cassette housing. The tape moves from one reel to the other during operations. A portion of the tape housing is typically designed to be opened automatically when the cassette is inserted into the tape drive, thereby exposing the tape. The drive read/write head, tape capstan spindle(s) and drive tape guides can then move into contact with the tape. In some cases, such as the VHS, the 8 mm and the 4 mm cassettes, a section of the tape is actually pulled out of the cassette during insertion into the drive and wrapped partly around the recording heads and over the drive tape guides.
Compared with the single reel cassette design, the dual reel cassette normally makes it possible to design a fairly simple drive mechanism, especially if the tape does not need to be partly pulled out of the cassette. Dual reel cassettes have two openings at the bottom where tape drive motors can engage to the tape reels inside the cassette. FIG. 2 shows the basic design principles of a dual reel cassette. A cassette housing 10 contains a tape 14 wound in reels 12 on respective hubs 11. A door 15 which opens during insertion into the drive to expose the front end of the cassette housing 11 and the tape 14.
A drawback of dual reel cassettes is that the there must be room for more than two full tape reels inside the housing. First of all, the distance from each hub to the inside of the cassette housing must at a minimum be equal to the maximum radius of a hub fully loaded with tape (plus a small tolerance/safety factor). Secondly, the distance between the hubs is a function of the diameter of a hub fully loaded with tape. It can be shown that the maximum distance required between the two hubs occurs when both hubs contain equal amount of tape(=half the tape length). If the diameter of a hub loaded with the whole tape length is D, then it can be shown that the minimum length between the two hubs must be D.sqroot.2 (assuming that the diameter of the empty hub is very small compared to the diameter of the full reel of tape). For example, if the maximum diameter of a full tape reel is 2" (50.8 mm), the minimum inside length of the cassette housing must be 2"+2"+1.4.multidot.2"=6.8" (172.7 mm). (As shown later, for practical designs, the distance between the hubs needs to be somewhat longer than the D.sqroot.2 length, because the diameter of the hub without any tape has to be taken into account).
By definition, within the same form factor, the dual cassette has a capacity for less tape media than a similar single reel cassette with the same form factor.
During recent years, the dual reel cassette has gained in popularity, especially because the mechanical design of the tape drive can be made far simpler and more reliable than for drives utilizing single reel cassettes. The fact that the available total length within a given form factor is typically 60 to 70% less than for a comparable single reel cassette, however, has hampered its acceptance in high end professional systems focusing on maximum capacity.
The belt driven cassette is a variant of the dual reel cassette. The most widely used version is the QIC (quarter inch cartridge) and its smaller companion, the Travan cartridge. FIG. 3 shows the basic principle of a conventional QIC cartridge. A drive puck 20 engages the capstan motor in the tape drive when the cartridge is loaded into the drive. The cartridge housing 21, a door 22 which opens during insertion to expose a portion of the tape 23, which is wound in tape packs 24 on respective hubs 25. A mirror 26 detects special holes at the beginning and end of the tape. A belt 28 is entrained around the drive puck 20 and friction rollers 28, and is in contact with both tape packs 24. When the drive motor rotates the drive puck 20, the belt 28 causes the tape 23 to be moved in a straight path defined by guide pins 29.
Compared to a dual reel cassette, the belt driven QIC cartridge is designed so that it is only necessary to use one motor driving the cartridge reels at one point (the drive puck 20 in FIG. 3). That can simplify the drive construction even further. Like the dual reel cassette, however, the belt driven QIC cartridge has the same limitations with respect to available tape length. Actually, because the belt driven cartridge typically needs to have some additional components (belt guides etc.) inside its housing, the available tape length may be less than for a similar dual reel cassette design.